Solar power (direct) -- a one that dwarfs all other ...
Transcript of Solar power (direct) -- a one that dwarfs all other ...
Solar power (direct) -- the “greenest” of all energy sources,
a one that dwarfs all other available
energy sources combined (including
those who are “transformed” forms
of solar energy).
Breakdown of the incoming solar energy
SOME IMPORTANT DEFINITIONS:
SOLAR CONSTANT: the total power of solar electromagnetic
radiation that falls on a unit surface area at a vertical angle
above the Earth atmosphere: s = 1.37 kW / m² (it’s the average
value)
The effective value at the Earth surface on a sunny day
is 0.8 – 1.0 kW / m².
INSOLATION ( not to be confused with insulation, or with
shoe insoles ): a measure of solar radiation energy received
on a given surface area in a given time. It is commonly expressed
as a 24-hour average irradiance in Watts per square meter (W/m2)
Annual insolation:
-- at the top of the
Earth atmosphere
-- at the Earth surface
Average insolation showing land area (small black dots) required to replace the world
primary energy supply with solar electricity. 18 TW is 568 Exajoule (EJ) per year.
Insolation for most people is from 150 to 300 W/m² or 3.5 to 7.0 kWh/m²/day.
Methods of harnessing solar energy:
We can divide them into two groups:
• The conversion of solar power
to electrical power;
• All other methods – many of
them are as old as human
civilization – for instance,
agriculture or horticulture,
or laundry drying. Rooftop
water heaters or sun-tanning :o))
are more recent inventions
We will focus on the first group of methods, i.e., on
generating “solar electricity”.
There are two major methods of
converting sunlight to electricity:
-- direct conversion using photovoltaic cell
panels; this method is used at all power
levels, from single household installations
up to multi-MegaWatt power plants.
-- methods known as “concentrating solar
power” (CSP) or “solar thermal conversion”.
Essentially, they are a combination of the old existing
technology (thermal engine, e.g., a steam turbine plus
a generator) with new methods of generating heat from
sunlight, not from fuel burning. CSP is used in the “high end”
installations (tens or even hundreds of MWs) and medium-size
ones, but not at the single-household level.
Since we already know a lot about thermal engines, we
will first discuss the “solar thermal conversion” methods.
Direct conversion
(a joke, of course)
Types of sunlight
collectors used in
CSP plants:
(a) Parabolic trough;
(b) The so-called
“Fresnel mirror”;
similar to (a), but
somewhat cheaper.
(c) Parabolic mirror
with a Stirling en-
gine (medium
power level);
(d) CSP tower tech-
nology, using flat
mirors and a heat
collector on a
tower top.
Explanation how a
parabolic trough
works.
A single parabolic
trough unit.
A large number of parabolic
troughs arranged into rows
in a multi-MW CSP plant.
The Fresnel mirror technology
CSP tower technology:
How it works
An existing power
plant. On a misty
day the beams from
individual mirrors
become visible,
producing a
spectacular
effect!
One highly attractive aspect of CSP power plants:
the capability of storing the daytime-collected
thermal energy.
There are several possible storage schemes – one
that is receiving strong attention is the use of huge
tanks filled with certain salts. The solar heat is
used for melting the salt. Later, during the heat
recovery phase, the salt solidifies, releasing a
considerable amount of latent heat (it’s called the
latent heat of fusion).
The process is analogous to ordinary water freezing,
in which a latent heat of 334 kJ/kg is given away by
liquid water – however, water freezes at 0 centigrades.
CSP plants with heat storage can generate power for
several hours after the sunset.
“old technology” part:
a steam turbine plus
Generator.
“new technology” part: solar
collectors and a tank system
for storing thermal energy.
Schematic of a CSP plant with energy storage capability
Concentrated Solar Power (CSP) plants
Concentrated Solar Power (CSP) plants convert thermal energy from the sun to electric
power the same way thermal power plants do – the only difference is that in the latter the
thermal energy is released by burning fuels.
In a CSP plant the sun rays are first focused, or “conce Archimedentrated”, on a heat-
collecting element. One method is to put a boiler on a high tower, and place a large number
of mirrors – called “heliostats” – on the field surrounding the tower. Each heliostat reflects
the solar rays and directs them to the boiler (the same way as we send away flashes of
sunlight using a “signal mirror” – as shown, e.g., in this Youtube clip; by the way, the thing
the gentleman holds is not a cell phone, but a framed rectangular mirror looking very much
like a cell phone).
By the way, the legend says that as many as 22 centuries ago, the Greek philosopher and
inventor Archimedes used reflections from many mirrors focused on Roman galleys to set
them afire – during the siege of the town Syracuse at Sicily by Roman fleet. At that time,
they did not have mirrors similar to these we use today – the legend says that Archimedes
used instead polished soldiers’ shields. This beautiful legend is over 1000 years old – but it
is only a legend, tests carried out at modern times with as many as 400 participant holding
replicas of highly polished ancient Greek shields showed that with such “heliostats” one
could lit a bonfire 50 yards away, but not an object half a mile away.
So, the modern CSP tower plants work very much the same way as Archimedes did in the
legend.
Concentrated Solar Power (CSP) plants with thermal storage
CSP plants have one very important “potential”. Namely, all energy collected during the
sunshine hours needs not to be used right away – some part of it may be stored to be
used later! After the sunset! It is very important! The hours after the sunset are still “peak
hours”, during which the demand for electric power is really high.
But how to store thermal energy? In principle, it’s not very difficult. Take a body, either a
solid or a liquid and warm it up with part of the energy collected during sunlight hours – and
then, after sunset, start taking this energy away, use it for making steam, send this steam
to the turbines, and you get your power.
Yet, there is a small problem… not even a small one. Namely, when you keep adding heat
to your “storage body”, its temperature gradually increases – it will be proportional to the
amount of the heat stored. In Week One, it was told that the amount of heat ∆Q transferred
to a body, and the resulting increase of the body’s temperature ∆T are related as:
∆Q = C•∆T,
where the coefficient C is the “heat capacity” of the body, depending of its mass and the
substance it is made of. Conversely,
∆T = ∆Q/C.
It means that if heat is gradually taken away from the body, its temperature gradually
decreases. And here is the problem! Why? Because all kind of thermal engines, steam
turbines included, are “very unhappy” if the temperature of the “hot source” from which they
draw thermal energy does change during the process they deliver work. An ideal situation
for them is if the temperature of the “hot source” remains constant as long as the engine
delivers power.
Fortunately, there is a remedy – one has to use as the “storage body” a substance
exhibiting a thermal process known as a “phase transition”.
A phase transition is something that you all know very well. Ordinary ice, as you know, is a
solid form of water, H2O – a “more professional” term used by physicists is solid phase.
Then, liquid water is the liquid phase. And what we call “steam” or “water vapor” is the
gaseous phase (or, more simply, gas phase) of water.
Any process in which water changes its phase from one to another is called a phase
transition. “Melting” is transition from a solid (ice) phase to a liquid phase (chemists call it
“fusion” – but not physicists). “Freezing”, or “solidification” is reverse transition, from a liquid
phase to a solid phase. “Evaporation” is transition from a liquid phase to a gas phase –
“boiling” is a fast evaporation process. And the reverse of evaporation is “condensation”.
An important fact is that each phase transition is associated with a transfer of heat, called
the latent heat of a phase transition. To melt one kilogram of ice, one needs to transfer 334
kilo-Joules of heat to it – in other words, the latent heat of ice melting is 334 kJ/kg.
Let’s consider the ice melting process in greater detail. Say, let’s begin with a one kilogram
sample of ice of temperature -10 C (= 263 K). We gradually deliver heat. The specific heat
of ice is 2.108 kJ/kg-K – it means that after delivering each portion of 2.108 kJ, the sample
temperature increases by 1C (= 1 K). And after we have delivered 21.08 kJ, the sample
temperature reaches 0C (= 273K), which is the melting point of ice.
We keep delivering heat – but now the temperature doesn’t grow any more, it stays at 0C.
However, the ice starts melting, more and more of it changes to liquid water. Yet, only after
we deliver as much as 334 kJ, all ice is melted and our sample is changed to liquid water –
now, if we keep delivering heat, the temperature starts rising again. The specific heat of
liquid water is 4.187 kJ/kg-K, so that now, after delivering each “heat portion” of 4.187 kJ,
the water temperature increases by another 1C.
If we cool down our water sample by taking heat away, the process is reversed. After
reaching the temperature of 0C “from above”, the water starts freezing – but its
temperature remains unchanged until we take away a total of 334 kJ of heat – at this
moment, all water is changed to ice and from now on, if we keep taking away heat from it,
its temperature will start decreasing.
Water is not good as a medium for “heat storage”, because it, yes, gives away heat – but
at 0C. So, why did we spent so much time to discuss it? Well, we did so because water
freezing and ice melting are phenomena that everybody knows very well. But now we need
to talk about other substances which are good candidates for heat-storing media. Let’s
specify what conditions such a medium must satisfy:
(a) melting temperature should be about the same as the typical temperature of steam
used by steam turbines;
(b) its latent head of melting/solidifying should be possibly high; and
(c) last but not least, it should be a relatively inexpensive substance.
It turns out that the best candidates are certain salts known of “nitrates”, of the formula
Me(NO3)N, where Me = an atom of a metal. So, we have LiNO3, NaNO3, KNO3,
Mg(NO3)2, Ca(NO3)2, and so on. LiNO3 has a record-high latent heat of solidifying, 360
kJ/kg, but one has to forget it because the price of Lithium is already high and probably
will become much higher in the near future (because it’s of crucial importance for electric
cars!). The latent heat of Magnesium and Calcium nitrates is not very high – but two
salts from the list, NaNO3 and KNO3, have both attractive melting temperature (306 and
337 C, respectively), and relatively high latent heat of solidifying (175 and 100 kJ/kg,
respectively). And they are inexpensive because they are widely used as fertilizers, and
therefore they are manufactured in large quantities ( after Google, the price per metric
ton of fertilizer-quality NaNO3 is $300-400, and of KNO3 is $700-800).
A list of CSP plants over the globe, active, under construction, and planned, is given in
this Wikipedia article. However, those with no storage capacity and those with such
capacity are listed together in the tables, so finding the ones with storage capacity
requires much patience.
Type equation here.
Latent heat storage – properties of some promising materials
For comparison:
Latent heat of fusion
for H2O (in other
words, of water
freezing) is 334 J/g
(or kJ/kg)
Another highly inte-
resting CSP techni-
que is the use of a
parabolic circular
mirror with a thermal
engine placed at its
focal point. Particu-
larly useful for this
are Stirling Engines
because of their
small size.
Such units typically
generate up to 10 kW
of electrical power,
so they should be classified as medium-power CSP
devices.
We will discuss the “strengths and weaknesses” of
the CSP plants after talking about the alternative
technology of generating solar electricity – namely,
by using panels of semiconductor solar cells.
I would enable us to compare the “plusses”
and “minuses” of the two methods.